1. Technical Field
The present invention relates to a variable delivery fuel supply device, which is used in an internal combustion engine for vehicles, supplies a pressure fuel to a fuel injection valve, and controls quantity of a fuel supplied to the fuel injection valve.
2. Background Art
A variable delivery fuel supply device for supplying a pressure fuel to a fuel injector of an internal combustion engine for vehicles is, as disclosed, for example, in the Japanese Patent publication (unexamined) No. 200990/1999, is comprised of: a fuel injection valve for injecting a fuel to each cylinder of the internal combustion engine; a delivery pipe (common rail) for distributing and supplying a pressure fuel to this fuel injection valve; a high pressure fuel pump for supplying the fuel to the delivery pipe; a low pressure fuel pump for supplying the fuel from a fuel tank to the high pressure fuel pump; and control means for controlling amount and time of injecting the fuel to each cylinder, controlling an electromagnetic valve (fuel pressure control valve) disposed in the high pressure fuel pump to release a part of the pressure fuel to a relief oil passage, and controlling discharge quantity of the high pressure fuel pump, thereby controlling fuel pressure of the delivery pipe.
The high pressure fuel pump is comprised of: a cylinder, a plunger which is driven by a cam disposed on a camshaft of the internal combustion engine to reciprocate in the cylinder, takes (sucks) the fuel into a pressurization chamber in the cylinder during an intake stroke while pressurizing the fuel in the pressurization chamber and delivering the pressurized fuel with pressure to the delivery pipe during a discharge stroke; the mentioned electromagnetic valve, and so on. This electromagnetic valve is a normally closed electromagnetic valve that is closed under normal conditions and is opened upon receiving an electric signal (valve-opening signal). The control means controls discharge quantity of the high pressure fuel pump by computing a discharge quantity from feedback of a fuel necessary for the fuel injection valve and a fuel pressure in the delivery pipe and determining a time for opening the electromagnetic valve. The electromagnetic valve is opened at the predetermined time, and the fuel pressure in the delivery pipe is maintained at a predetermined value by relieving a part of the pressure fuel to the relief oil passage. A valve-opening signal is given from the control means to the electromagnetic valve in the discharge stroke of the plunger of the high-pressure fuel pump. A signal width of the valve-opening signal is determined so that the valve may be closed upon completion of the discharge stroke, i.e., along with the beginning of the intake stroke, or during the intake stroke.
FIG. 7 is a diagram to explain signal width of the valve-opening signal and an operating condition of the high-pressure fuel pump in the mentioned conventional device. The drawing shows one reciprocating motion, i.e. operation in one cycle, and in the case that the cam has, for example, four tops, the drawing shows the operation during a quarter of one revolution (90xc2x0 in rotational angle) of the cam. It is understood from the drawing that, in one cycle, the discharge stroke occupies 45xc2x0 and the intake stroke occupies 45xc2x0. When the discharge quantity reaches a predetermined value at a point (a) in the discharge stroke, a valve-opening signal is given to the electromagnetic valve, and the fuel in the pressurization chamber is relieved to the relief oil passage during the period from the point (a) to the top dead point. Controlling the position of the point (a) controls the discharge quantity. When a flow rate required by the control means is large with respect to the discharge quantity of the high pressure fuel pump, for example, when an amount of fuel used by the internal combustion engine is large or when fuel pressure of the delivery pipe is increased from a low pressure to a high pressure such as at the time of starting, the electromagnetic valve is kept closed until coming near the top dead point.
FIG. 8 shows an amount of lift of the plunger (hereinafter referred to as plunger lift amount) with respect to the rotational angle of the cam in such operation, and is an enlarged diagram of the mentioned 45xc2x0 from the bottom dead point to the top dead point of the plunger lift amount shown in FIG. 7. A curve indicated by the solid line in the drawing is a lift curve of the cam shown in the form of a sine curve. The fuel discharge quantity with respect to the cam angle in a predetermined number of revolutions in this case becomes a discharge characteristic curve as indicated by the solid line in FIG. 9. The discharge flow rate with respect to the valve-opening time (shown in terms of rotational angle of the cam) is lowered more in linearity as the plunger approaches the top dead point. In the operation, the pressure in the pressurization chamber generates a Hertz stress between the cam and the plunger. The solid line in FIG. 10 indicates the Hertz stress with respect to the cam angle under a predetermined discharge pressure. When the lift of the cam is a sine curve, the Hertz stress sharply increases because the radius of curvature of the cam becomes small in the vicinity the top dead point of the plunger.
Such increase in Hertz stress brings about an abrasion on the contact surface of the cam, and this abrasion changes the discharge quantity. Therefore, the lift curve of the cam is changed into a lift curve that is large in radius of curvature in the vicinity of the top dead point (the cam top portion of the cam) as indicated by the dotted line in FIG. 8 to lower the Hertz stress which is a counter-measure to the abrasion. The discharge flow rate characteristic and the Hertz stress at this time are as indicated by the dotted lines in FIGS. 9 and 10. It is certain that the Hertz stress in the vicinity of the plunger top dead point is lowered. But, on the contrary, linearity of the discharge flow rate is lowered. Assuming that the top dead point side of the discharge stroke is 100%, increase in discharge quantity with respect to the rotational angle of the cam becomes extremely small in the range of approximately 80% to 100% of the discharge stroke. In such a range controllability is deteriorated remaining only a large Hertz stress.
As discussed above, in the mentioned conventional variable delivery fuel supply device, the whole period of the discharge stroke is occupied by the valve-closing time of the electromagnetic valve under the condition that fuel consumption is close to the discharge quantity. As a result, a problem exists in that the Hertz stress increases in the vicinity of the top dead point leading to an abrasion on the contact face of the cam, and if such an abrasion develops further, the lift of the plunger is lowered and the discharge quantity becomes poor. And the linearity of the discharge characteristic lowers and the controllability of the discharge flow rate is deteriorated as the plunger approaches the top dead point. Further, in the conventional example, since the pressure in the pressurization chamber is forcibly lowered to an intake pressure in a short time at the time of opening the electromagnetic valve under normal conditions, the intake valve is opened when the plunger moves downward during the intake stroke. On the other hand, since the valve is kept closed throughout the whole stroke of the discharge stroke and the intake stroke at the time of obtaining the maximum discharge quantity, it takes a certain time to lower the fuel pressure in the pressurization chamber even after the stroke is shifted to the intake stroke and the plunger begins to move downward because of bulk modulus of the fuel. As a result, a further problem exists in that there is not enough time for sufficiently taking in the fuel under the conditions of high numbers of revolution and a high fuel pressure, which brings about cavitation and deterioration in durability.
Moreover, the lift curve is changed as a counter-measure to the abrasion of the cam due to the Hertz stress in the foregoing description, and this change is, as described above, to lower the Hertz stress by increasing the radius of curvature of the cam top portion. In this case, to assure the amount of lift of the cam, it is necessary to decrease curvature of a cam base portion, which means that a component of higher order is added to the lift curve in the form of sine curve only formed of primary components. In this known measure, it is certainly possible to lower the Hertz stress without enlarging external diameter of the cam. But in the case that, for example, a cam has four tops and speed of revolution of the cam is 3500 r.p.m., the high pressure fuel pump is driven at a frequency of 233 Hz, while the drive frequency of the high pressure fuel pump includes a high frequency of 466 Hz when a component of higher order, for example, any secondary component is added. As a result, a further problem exists in that it is essential to establish resonance point of the intake valve, discharge valve, plunger, spring, and so on, to be higher.
The present invention was made to resolve the above-discussed problems and has an object of obtaining a variable delivery fuel supply device in which it is possible to improve linearity of discharge characteristic with respect to a rotational angle of a cam and decrease abrasion and cavitation of the cam.
A variable delivery fuel supply device according to the invention comprises fuel injection valves respectively for injecting a fuel to cylinders of an internal combustion engine; a delivery pipe for supplying a pressurized fuel to the mentioned fuel injection valves; and a fuel pump for taking a fuel from a fuel intake passage through an intake valve into a pressurization chamber during an intake stroke, for pressurizing the fuel and for discharging the pressurized fuel through a discharge valve to the delivery pipe during a discharge stroke by reciprocating movement of a plunger in each cylinder from a bottom dead point to a top dead point; wherein the pressure in the pressurization chamber is relieved at a position set to a predetermined value before reaching the top dead point in the course of the discharge stroke in which the plunger of the fuel pump moves from the bottom dead point to the top dead point.
As a result, the pressure in the pressurization chamber is relieved at the time of completion of the discharge stroke and shifting to the intake stroke is performed smoothly. It is possible to prevent deterioration in durability due to cavitation or the like and, at the same time, solve a problem of insufficient filling with the fuel at high numbers of revolutions or at a high load. Further, the pressure applied to the plunger is lowered, and consequently, the Hertz stress acting between a cam top portion of a small radius of curvature of the cam and a roller is largely reduced. Thus it is possible to prevent the cam from abrasion.
It is also preferable that the variable delivery fuel supply device is provided with an electromagnetic valve for controlling a discharge quantity of the fuel pump to control the fuel pressure of the delivery pipe by connecting the fuel intake passage and the pressurization chamber, and the pressure in the pressurization chamber is relieved by opening the electromagnetic valve at a set position.
As a result, it is possible to release the pressure just by changing the control contents of the electromagnetic valve for controlling the discharge quantity.
It is also preferable that the position of the plunger where the pressure in the pressurization chamber is relieved is established to be at 80 to 90% from the bottom dead point side in the whole stroke of the plunger from the bottom dead point to the top dead point.
As a result, it becomes possible to assure linearity of the discharge quantity with respect to the rotational angle of the cam during the period of discharge and improve accuracy in discharge quantity. Furthermore, just by changing a small diameter portion of the cam and changing the plunger lift amount to 1/(0.8 to 0.9) times, it becomes possible to secure the whole discharge quantity of the fuel pump without increasing size, and decrease in discharge quantity is compensated by regulating the discharge control range to meet the abrasion of the cam. Consequently, it is possible to obtain a long-life variable delivery fuel supply device.
It is also preferable that the electromagnetic valve is a normally closed valve, which opens when the discharge quantity is controlled and when the pressure in the pressurization chamber is relieved.
As a result, the pressure in the pressurization chamber is relieved easily, and it is possible to supply a fuel to each cylinder even when any trouble occurs in the drive circuit of the electromagnetic valve and prevent the device from becoming inoperative.